A sintered compact of silicon nitride (hereinafter called a "sintered compact of Si.sub.3 N.sub.4 " for ease of reference) is known not only as a material having excellent oxidation resistance but also as a material having a low coefficient of thermal expansion and intensive strength properties at high temperatures. Moreover, research and development activities are being conducted to enable sintered compacts of Si.sub.3 N.sub.4 to be utilized as a high-temperature structural material for turbine engine blades and nozzles and for heat exchange members, to name a few.
However, because powder metallurgy is normally employed to produce sintered compacts of Si.sub.3 N.sub.4, it is difficult to obtain sintered compacts having complicated shape configurations, accurate dimensions and planes. Finished shaped products of sintered compacts of Si.sub.3 N.sub.4 are therefore typically produced by machining, or grinding after sintering.
As is commonly known, a sintered compact of Si.sub.3 N.sub.4 is a very hard material and thus very difficult to machine. Consequently, development in the field of sintered compact applications has been hampered by the technical restrictions imposed due to the difficulties of machining sintered compacts of Si.sub.3 N.sub.4. Such technical restrictions include, for exmple, a large amount of time and labor required even if such machining is feasible; only relatively simple shape configurations are available with such machining; and particularly, thin parts such as turbine blades typically cannot be produced.
Electric discharge machining is generally known as one of the means for machining into finished parts having complicated shape configurations. However, sintered compacts of Si.sub.3 N.sub.4 conventionally produced are electrically insulative and thus have not been conventionally thought of as being suitable for electric discharge machining which requires the compact to be electrically conductive.
According to the present invention, however, there has been obtained a conductive sintered compact of Si.sub.3 N.sub.4 which is machinable by electric discharge machining. The present invention is realized by the addition of powders including a conductivity-supplying agent and a sintering assistant to the Si.sub.3 N.sub.4 powder. The resulting Si.sub.3 N.sub.4 powder is then sintered so that the excellent properties associated with conventional sintered compacts of Si.sub.3 N.sub.4 are maintained while yet producing a sintered compact of Si.sub.3 N.sub.4 capable of being machined by electrical discharge machining techniques.
More specifically, TiN and/or TiC powders are employed as conductivity-supplying agents and MgO and/or Al.sub.2 O.sub.3 powders are employed as sintering assistants. The TiN and/or TiC in addition to the MgO powders are crushed to particles measuring 2 .mu.m or smaller in average size before being uniformly dispersed in the Si.sub.3 N.sub.4 powder and shaped into a compact preform. The resulting preform is then subjected to hot isostatic pressing at an elevated temperature within the range 1,600.degree. to 2,000.degree. C. in a nonoxidizing atmosphere to obtain a sintered compact of Si.sub.3 N.sub.4 which has a conductivity of 1 S.multidot.cm.sup.-1 or greater and is machinable by electrical discharge machining.
TiN and/or TiC powders are utilized as a conductivity-supplying agent due to their high electrical conductivity (i.e., electrical conductivity of TiN is 4.times.10.sup.4 S.multidot.cm.sup.-1, while that of TiC is 3.times.10.sup.4 S.multidot.cm.sup.-1) which is substantially equivalent to that of metal, their greater hardness and their stability at high temperature. The use of MgO and/or Al.sub.2 O.sub.3 on the other hand are used as sintering assistants since addition of a small amount of either is not only effective for the sintering of a Si.sub.3 N.sub.4 matrix but also contributive to the sintering of TiN and/or TiC.
The percentages of the conductivity-supplying agent and the sintering assistant to be added should preferably be 15 to 40% by volume and 0.01 to 3% by volume, respectively, and the percentage should be determined in consideration of the following points. First, while TiN and TiC as conductivity-supplying agents both exhibit excellent stability at high temperatures, they are both less stable than Si.sub.3 N.sub.4. Thus, the amount of the TiN and/or TiC additive should be minimized to the extent that satisfactory properties for electrical discharge machining are obtainable. Secondly, the amount of the sintering assistant to be added should also be minimized to the extent that a high sintering density can be obtained.
As described later, the addition of a large amount of the sintering assistant will be followed by excessive growth of Si.sub.3 N.sub.4 particles, whereby the conductivity-supplying agent and the electrical conductivity (that is, the properties for electrical discharge machining and oxidation resistance at high temperature) will be deleteriously affected.
Further aspects and advantages of the present invention will become more clear after consideration is given to the detailed description in conjunction with the examples.